The present invention relates to a diaphragm type air pump used in an automotive vehicle for supplying air as a secondary air to an exhaust system of the engine for the purpose of reducing harmful components in the exhaust gases emitted from the engine.
A vane type air pump is generally and conventionally used for supplying the secondary air to the exhaust system of the engine, which is driven by the engine through a pulley fixed to an engine crankshaft and a belt. In recent years, however such a method or system has been proposed and widely used, which draws air into the exhaust pipe through a check valve such as a reed valve as the secondary air as a function of negative pressures produced in the exhaust pipe because of pulsating exhaust back pressures. Because the engine is supplied with and driven by lean air-fuel mixtures from view points of emission control and fuel consumption, and thereby the amount of the secondary air drawn into the exhaust system through the check valve is sufficient for the purpose of reducing the harmful components.
However, since the amount of the air drawn into the exhaust pipe through the reed valve is dependent on the negative pressures caused by the pulsating back pressures in the exhaust pipe, it is affected by the opening and closing timings of the exhaust valve, the shape and dimension of the exhaust pipe and so on. So some types of engines may run short of the secondary air. Such types of engines therefore require means for compulsorily feeding air to the exhaust pipe, even though the engines are operated with lean air-fuel mixtures.
When the conventional vane type air pump is used in such engines operated with lean mixtures, the capacity of the pump is too large. On the other hand, the vane type air pump has a disadvantage in that the smaller the capacity of the pump is made, the worse the efficiency becomes.
In view of the above problems, a diaphragm type air pump has been proposed instead of the vane type one. However, since the conventional air pump of the type is driven by the engine through the pulley and belt, it is required to install the pump in the engine compartment at such a place where the pulley of the engine crankshaft and the pulley of the pump are coplanar. On the other hand, many other automotive engine equipments such as an alternator, a radiator fan, a compressor for air conditioners and so on are also necessary to be installed in the engine compartment in the coplanar places with the pulley of the engine crankshaft.
It is therefore pretty hard for the air pump driven by the engine through the pulley and belt to provide a space therefor in the engine compartment, and it is disadvantageous in that the provision of the air pump increases flow resistance of cooling air for the radiator with the resultant increase of the temperature in the engine compartment.